1. Field of the Invention
The present invention relates to a display device wherein after the process of manufacturing a cell, a solderable terminal of a thick film conductor is formed on a transparent electrode of a display panel by using a dry or wet transfer mounting method so that the display panel is connected to a driver circuit mounted on a flexible film; a process for producing such a display device; and a decal for forming a display panel terminal.
2. Discussion of the Background
FIG. 1 shows a cross sectional view of a conventional display device, particularly a liquid crystal display device. Upper and lower base plates 1 and 2 having electrodes 4 made of a transparent conductive coating film are bonded under pressure to each other with a space of from 5 to 10 .mu.m by a sealing material 3 and sealed by e.g. heating or ultraviolet irradiation. Then, a liquid crystal material 5 is filled through a liquid crystal inlet formed on the side surface of the sealing material 3 or on the lower base plate 2, and the liquid crystal inlet is then sealed by a sealing material such as a resin. If necessary, additional elements such as polarizer 6, a color polarizer, a color filter, an ultraviolet absorbing filter, a reflector 7, a light-conductive plate, a quarter wave plate or a light source, may be laminated, and various printed patterns or non-glare treatment may be applied to the surface. A double-layered structure may be employed for the liquid crystal layer or for the electrode layer. The lower base plate 2 may be made of a reflective base plate or a semiconductor substrate. A predetermined voltage is applied to the electrodes of the liquid crystal display panel thus prepared, to display predetermined letters, characters or patterns. Various modes for the display are available such as a dynamic scattering mode, a field effect mode or a guest-host mode.
The overall structure of a liquid crystal display device comprises the above-mentioned liquid crystal display panel and a driver circuit connected thereto. Namely, an IC chip 9 is mounted on a printed circuit board 8. In most cases, a conductive elastomer 10 is disposed between the terminal portion of transparent electrode of the liquid crystal display panel and the terminal portion of the print circuit board so that the electrodes are mutually connected by the conductive elastomer 10. However, in the display device wherein the conductive elastomer 10 is employed, the display panel is not securely fixed to the print circuit board since the device is fabricated by closely spaced contacts, whereby it is likely that the conductive elastomer 10 moves and brings about a failure in the connection. Further, a failure in the electrical conduction is likely to be led by the deformation of the conductive elastomer 10, caused by closely spaced contacts, whereby an additional step for correcting the failure is required.
These problems increase as the size of the panel increases, and a new technique for the processing is required.
Furthermore, with respect to the design of the display device, when a conventional conductive elastomer is used, there is a drawback that the display area is restricted by the holder material for the conductive elastomer. These problems are attributable to closely spaced contacts for electrical connection between the display panel and the drive circuit. Therefore, it is desirable to employ a bonding method wherein the electrode terminal of the display panel and the driver circuit are connected by soldering.
Methods for forming a terminal portion of electrode of the panel include (1) a thick film method by screen printing (i.e. a direct printing method), (2) a thin film method by vapour deposition or sputtering, (3) a plating method, and (4) a dispenser method with a conductor paste (i.e. a coating method).
However, none of these methods is totally satisfactory since each method has its own demerit as described below.
Namely, in the thick film method by screen printing (direct printing method), when the thickness of a seal to connect the upper and lower base plates of the display panel is from 5 to 10 .mu.m and the thickness of the conductor at the terminal to be formed is from 12 to 20 .mu.m, the resulting printed sheet material will tend to warp. Consequently, when the sheet material is cut into separate pieces, the respective base plates are likely to be cracked or broken, or due to the warping, the cell gap of the liquid crystal display cell tends to be locally widened, whereby the response speed, the legibility or the outer appearance will be impaired. Furthermore, for the same reasons as mentioned above with respect to the cell gap, it becomes difficult to employ a conventional method to obtain a plurality of liquid crystals display cells by cutting them out from the sheet material. Thus, it is thereby difficult to obtain a desired cell gap, whereby the productivity is remarkably lowered.
The thin film method requires a step of vapour deposition or sputtering, whereby the productivity will be lowered and the cost will be increased. Moreover, there will be a problem such as corrosion of the transparent conductive coating film due to pin holes or scars which are likely to be occurred thereon.
In the plating method, the adhesive strength of the deposited film is inadequate in many cases. According to the dispenser method, the dimensional precision is poor, and it is difficult to form fine patterning.